The present invention relates to a method and apparatus for testing the linearity of a digital-to-analog converter.
Digital-to-analog converters, also referred to as D/A converters or DACs, are manufactured in large quantities for use in applications ranging from motor control to the synthesis of audio and video signals. DACs are classified according to the number of bits of digital input they convert, e.g. a five-bit DAC accepts five-bit digital input.
Ideally, the output of a DAC varies linearly between a minimum value and a maximum or full-scale value. Nonlinearity, also referred to as integral nonlinearity, can be defined as the maximum deviation of the actual output from the ideal output. Nonlinearity is an important performance parameter. DAC manufacturers often guarantee that the nonlinearity of their products will not exceed a specified limit, and test their products to ensure compliance with this limit.
A conventional test method supplies all digital input values in sequence to the DAC and measures the corresponding output values. A problem with this conventional method is that testing an n-bit DAC requires 2.sup.n separate measurements. Testing a five-bit DAC, for example, requires thirty-two measurements, while a ten-bit DAC requires one thousand twenty-four measurements. Even with a computer-controlled test and measurement system, performing this large a number of measurements takes time, so that nonlinearity testing becomes expensive and impedes productivity.